The present invention relates to a fluid pressure boosting device, which hydraulically boosts input into predetermined magnitude, and a brake pressure boosting device employing the device. More particularly, the present invention relates to a fluid pressure boosting device, which can vary the servo ratio during the servo control and a brake pressure boosting device employing the device.
The fluid pressure boosting device employed in a brake pressure boosting device of the type utilized in automotive vehicles functions to obtain large output from small input. As an example of the fluid pressure boosting device, a brake pressure boosting device employed in a brake system of an automobile is proposed in Japanese Utility Model Application No. 4-33402 (Japanese Utility Model Unexamined Publication No. 5-84553) which is stored on a microfilm.
FIG. 52 is a view showing the brake pressure boosting device disclosed in this microfilm. In this figure, numeral 1' designates a brake pressure boosting device, 2' designates a housing, 3' designates a plug, 4' designates a power piston, 5' designates a control valve, 6' designates a valve seat, 7' designates a cylindrical fixing member, 8' designates a nut, 9' designates a valve ball, 10' is a valve body, 11' designates a cylindrical member, 12' designates an input shaft, 13' designates a cylindrical stopper, 14' designates a reaction piston, 15' designates a power chamber, and 16' designates an output shaft.
In the brake pressure boosting device 1', in the non-operation state as illustrated, the valve ball 9' of the control valve 5' is seated in the valve seat 6' and a valve portion at the end of the cylindrical member 11' is separated from the valve ball 9'. Therefore, the power chamber 15' is shut off from an input port 17', which always communicates with the fluid pressure source not shown, and communicates with a chamber 18', which always communicates with a reservoir not shown. Accordingly, no fluid pressure is introduced in the power chamber 15' so that the power piston 4' is not actuated.
As input is applied from this non-operation state, the input shaft 12' advances and the cylindrical member 11' also advances. The valve portion at the end of the cylindrical member 11' therefore comes into contact with the valve ball 9' of the control valve 5' and pushes the valve ball 9' so that the valve ball 9' is separated from the valve seat 6'. Accordingly, the power chamber 15' communicates with the input port 17' and is shut off from the chamber 18' so that fluid pressure is introduced into the power chamber 15' and the power piston 4' is thus actuated. By the actuation of the power piston 4', the brake pressure boosting device 1' produces output through an output shaft 16' to actuate a piston of a master cylinder not shown so that the master cylinder produces brake pressure. As the fluid pressure in the power chamber 15' reaches the magnitude corresponding to the input, the valve ball 9' is seated in the valve seat 6', so the output of the brake pressure boosting device 1' becomes a boosted value relative to the input.
By the fluid pressure in the power chamber 15', a reaction piston 14' is pushed rearwardly against the biasing force of a spring 19'. In the initial stage, the fluid pressure in the power chamber 15' is still small and play in stroke of the brake system is not yet cancelled so that substantially no braking force is developed. In this state, the reaction piston 14' is not in contact with a stepped portion 12'a of the input shaft 12'. Therefore, jumping action by the servo control with quite high boosting ratio, i.e. servo ratio, is performed. When the fluid pressure in the power chamber 15' reaches the predetermined pressure and the reaction piston 14' comes into contact with the stepped portion 12'a of the input shaft 12', braking force is then substantially produced and the servo ratio is reduced to normal servo ratio. After that, the brake pressure boosting device 1' performs the servo control for normal braking to produce output which is a boosted force relative to the input at the servo ratio.
The fluid pressure in the power chamber 15' reaches the maximum pressure, which is decided based on the pressure produced from the fluid pressure source, so the fluid pressure no more increases. The brake pressure boosting device 1' is fully loaded and does not perform the servo control. Therefore, after that, the output increases in proportion to the input without magnification.
As the input is cancelled, the input shaft 12' is moved rearwardly by a return spring not shown and the cylindrical member 11' is also moved rearwardly so that the valve portion at the end of the cylindrical member 11' is spaced apart from the valve ball 9' of the control valve 5'. As a result of this, the power chamber 15' is shut off from the input port 17' and communicates with the chamber 18' so that the fluid introduced into the power chamber 15' is discharged to the reservoir and the power piston 4' is moved rearwardly by the return spring 20'. As the cylindrical stopper 13' fixed to the input shaft 12' comes into contact with a stopper 21' of the plug 3', the input shaft 12' is moved to the rearmost position where it is no more moved rearwardly, that is, the input shaft 12' returns to the non-operation state as illustrated. As the fluid in the power chamber 15' is completely discharged, the power piston 4' also returns to the non-operation state as illustrated so that the brake pressure boosting device 1'outputs nothing and the master cylinder also returns to the non-operation state.
In the conventional brake pressure boosting device 1', the servo ratio of the servo control is constant while braking forces are substantially outputted as shown in FIG. 53. The servo ratio is normally set to correspond to the input-output characteristic which is desired in normal braking. Since the servo ratio is constant, the input-output characteristic for emergency braking should be the same as that of the normal braking. That is, even for normal braking or emergency braking, the braking force is always increased at the same speed as long as the same input is applied.
In a brake system for a vehicle, however, it is desired that, in case of emergency braking, the boosting device quickly produces braking force larger than that in case of normal braking. Among inexpert drivers, there must be a person who can not pedal a brake pedal fully so that it can develop large braking force. In this case, it is desired to provide brake assist for ensuring that large braking force is obtained even by such an inexpert driver.
In the conventional brake pressure boosting device 1', however, since the servo ratio of the servo control is constant while braking forces are substantially outputted, braking force larger than that of normal braking can not be produced for emergency braking, but also no braking assist for ensuring that large braking force is obtained even by the inexpert driver is provided. Therefore, it is difficult to meet the aforementioned demands.
In addition, as the servo ratio is constant, the output is always increased at the same rate relative to the increase in the input even after the braking force is increased relatively, so the braking force is apt to become larger than necessary. This should give the driver a quite bad operational feeling. Therefore, it is preferable that, after the braking force is increased relatively, the increase in the output is small relative to the increase in the input to prevent the braking force from being larger than necessary, thereby improving the operational feeling.
Among conventional brake pressure boosting systems, a brake pressure boosting system has been developed which is provided with Anti-Lock Control (hereinafter, sometimes referred to as ABS) for controlling braking pressure of a wheel cylinder when the a braked wheel is in locking tendency in order to cancel the locking tendency, Traction Control (hereinafter, sometimes referred to as TRC) for controlling the rotational driving force of driving wheels by automatically braking the driving wheels when the driving wheels are in slipping tendency in order to cancel the slipping tendency, Vehicle Stability Control (hereinafter, sometimes referred to as VSC) for controlling the attitude of a vehicle by automatically braking inside wheels when the vehicle turns, and Auto Cruise Control (hereinafter, sometimes referred to as ACC) for controlling the vehicle to run at a constant speed by automatically braking wheels.
Also in the above brake pressure boosting system, that brake assist is preferably provided for securely producing large braking force. In this case, it is preferable to utilize conventional parts as much as possible not to require any special parts for the brake assist.
The conventional brake fluid pressure boosting device 1' does not have hysteresis, that is, in the servo-ratio characteristic, the line in the operative direction and the line in the operation release direction coincide with each other. Accordingly, also in the brake characteristic thereof, the line in the operative direction and the line in the operation release direction always coincide with each other.